Robotic radiosurgery treatment in liver tumors: Early experience from an Indian center.

PURPOSE: The purpose of this study is to report CyberKnife experience in hepatocellular carcinoma (HCC) and liver metastasis (LM). MATERIALS AND METHODS: Fifty liver lesions in 31 consecutive patients with liver lesion [mean age 54.5 years (range 32-81 years), 77% were male patient, GTV <10cc in 5 patients, 11-90cc in 18 & >90cc in 8 patients respectively. Eighty percentage (25/31) had prior treatment (chemotherapy 18 patient & TACE in 7 patients). Dosage schedule was 21-45Gy/3# (mean PTV dose 33Gy, Prescription isodose 84%, target coverage 94%). Mean CI, nCI & HI were 1.19, 1.31 & 1.18 respectively. Mean liver dose was 5.4 Gy, 800 cc liver dose 11.1 Gy. RESULTS: At mean follow-up of 12.5 months (range 1.9-44.6 months), 19 patients were expired and 12 were alive (nine patient with stable disease, two local progression, and one with metastasis). Median overall survival (OS) of all patients are 9 months (1.9-44.6 months), in HCC patients 10.5 months (2.1-44.6 months) and MT 6.5 months (1.9-24.6 months) respectively. Gr-I-II GI toxicities were in 11/50 (22%) patients. OS was influenced by PS (Karnofsky Performance Status 70-80 vs. 90-100: 9.9 vs. 16.4; P = 0.024), Child-Pugh (CP A/B vs. C: 23.6 vs. 6.5; P = 0.069), cirrhosis (only fatty liver vs. diffuse cirrhosis: 17.8 vs. 10.6; P = 0.003), prior treatment (no Rx vs. prior Rx: 30.1 vs. 8.2; P = 0.08), number of lesions (single vs. multiple: 16.4 vs. 6.9; P = 0.001), and target volume (<10 cc vs. >90 cc: 24.6 vs. 11.2; P = 0.03). CONCLUSION: Stereotactic body radiation therapy is a safe and effective treatment. Patient related factors such as performance status, Child-Pugh classification, cirrhosis status, prior treatment, number of liver lesion & target volume (GTV) influence the survival functions.

Comparative analysis between 5 mm and 7.5 mm collimators in CyberKnife radiosurgery for trigeminal neuralgia.

Trigeminal neuralgia (TN) is treated in CyberKnife (Accuray Inc, Sunnyvale, USA) with the 5 mm collimator whose dosimetric inaccuracy is higher than the other available collimators. The 7.5 mm collimator which is having less dosimetric uncertainty can be an alternative for 5 mm collimator provided the dose distribution with 7.5 mm collimator is acceptable. Aim of this study is to analyze the role of 7.5 mm collimator in CyberKnife treatment plans of TN. The treatment plans with 5 mm collimators were re-optimized with 7.5 mm collimator and a bi-collimator system (5 mm and 7.5 mm). The treatment plans were compared for target coverage, brainstem doses, and the dose to normal tissues. The target and brainstem doses were comparable. However, the conformity indices were 2.31 ± 0.52, 2.40 ± 0.87 and 2.82 ± 0.51 for 5 mm, bi-collimator (5mm and 7.5 mm), 7.5 mm collimator plans respectively. This shows the level of dose spillage in 7.5 mm collimator plans. The 6 Gy dose volumes in 7.5 mm plans were 1.53 and 1.34 times higher than the 5 mm plan and the bi-collimator plans respectively. The treatment time parameters were lesser for 7.5 mm collimators. Since, the normal tissue dose is pretty high in 7.5 mm collimator plans, the use of it in TN plans can be ruled out though the treatment time is lesser for these 7.5 mm collimator plans.

Analysis of high-dose rate brachytherapy dose distribution resemblance in CyberKnife hypofractionated treatment plans of localized prostate cancer.

The present study is to analyze the CyberKnife hypofractionated dose distribution of localized prostate cancer in terms of high-dose rate (HDR) brachytherapy equivalent doses to assess the degree of HDR brachytherapy resemblance of CyberKnife dose distribution. Thirteen randomly selected localized prostate cancer cases treated using CyberKnife with a dose regimen of 36.25Gy in 5 fractions were considered. HDR equivalent doses were calculated for 30Gy in 3 fractions of HDR brachytherapy regimen. The D5% of the target in the CyberKnife hypofractionation was 41.57 ± 2.41Gy. The corresponding HDR fractionation (3 fractions) equivalent dose was 32.81 ± 1.86Gy. The mean HDR fractionation equivalent dose, D98%, was 27.93 ± 0.84Gy. The V100% of the prostate target was 95.57% ± 3.47%. The V100% of the bladder and the rectum were 717.16 and 79.6mm(3), respectively. Analysis of the HDR equivalent dose of CyberKnife dose distribution indicates a comparable resemblance to HDR dose distribution in the peripheral target doses (D98% to D80%) reported in the literature. However, there is a substantial difference observed in the core high-dose regions especially in D10% and D5%. The dose fall-off within the OAR is also superior in reported HDR dose distribution than the HDR equivalent doses of CyberKnife.

Monte Carlo and ray tracing algorithms in the cyberknife treatment planning for lung tumours- comparison and validation.

Multiplan treatment planning system, used with Cyberknife system, provides the option of using either the ray tracing algorithm or the Monte Carlo algorithm for the final dose calculation. In order to compare and validate the dose calculations of these algorithms, especially in a heterogeneous medium, a lung phantom study was carried out. Validation has been done with thermoluminiscent dosimetry (TLD) using lithium fluoride rods for the point doses and film dosimetry using EBT2 films for the dose distribution. In the point dose measurements, an agreement of 100.1+2.6 % (1 SD) is observed with the Monte Carlo dose calculation, whereas it is only 91.2+ 3.2% (1 SD) with the ray tracing calculation. On subjecting the dose distributions from irradiated EBT2 films for validation of Monte Carlo calculation MC , over 96% of the pixels pass the gamma criteria of 3mm and 3cGy.On analyzing the dose profiles from EBT2 films and the corresponding profiles from the plan calculated using the Monte Carlo algorithm, it is seen that the maximum distance-to-agreement values are within the 3mm criteria set, whereas the maximum values are as high as 8 mm when compared with plan calculated using ray tracing algorithm. The results of the actual measurements are more consistent with the dose calculation by the Monte Carlo algorithm.

Influence of smoothing algorithms in Monte Carlo dose calculations of cyberknife treatment plans: a lung phantom study.

AIM: The Monte Carlo dose calculation algorithm yields accurate dose distributions in heterogeneous media and interfaces. The Monte Carlo calculation algorithm provided in the Multiplan Cyberknife treatment planning system (Accuray, Sunnyvale, CA, USA) has five different dose-smoothing algorithms in it. As the principle of smoothing of these algorithms is different, they can produce a disparity in the final dose distribution. The aim of the present study is to analyze the influence of these Monte Carlo smoothing algorithms in the final dose distribution of cyberknife treatment plans. MATERIALS AND METHODS: An anthropomorphic lung phantom with a tumor mimicking ball target was taken for this study. The basic optimization was performed with the Ray tracing algorithm. The Monte Carlo calculations were introduced with each smoothing algorithm on the basic plan and the plans were compared. RESULTS: The Monte Carlo doses were found to be lesser than the Ray tracing doses. The dose conformity index was above 4 for all the smoothing algorithms, while it was only 1.19 for Ray tracing. The least coverage of 6.34 was obtained for a weighted average algorithm. The deviation between the V100% values of different smoothing algorithms was higher than the deviation in V80%. CONCLUSION: The deviations between the smoothing algorithms are higher in the high-dose regions, including the prescribing isodose, than the low-dose regions of the target, as well as in the organs at risk (OAR).

Dosimetric analysis of trigeminal nerve, brain stem doses in CyberKnife radiosurgery of trigeminal neuralgia.

CyberKnife radiosurgery treatment of Trigeminal neuralgia (TN) is performed as a non-invasive image guided procedure. The prescription dose for TN is very high. The brainstem is the adjacent critical organ at risk (OAR) which is prone to receive the very high target dose of TN. The present study is to analyze the dose distribution inside the tiny trigeminal nerve target and also to analyze the dose fall off in the brain stem. Seven TN cases treated between November 2010 and January 2012 were taken for this study retrospectively. The treatment plans were analyzed for target dose conformity, homogeneity and dose coverage. In the brainstem the volume doses D(1%), D(2%) were taken for analyzing the higher doses in the brain stem. The dose fall off was analyzed in terms of D(5%) and D(10%). The mean value of maximum dose within the trigeminal nerve target was 73.5±2.1Gy (P=0.0007) and the minimum dose was 50.0±4.1Gy (P=0.1315). The mean conformity index was 2.19 and the probable reason could be the smallest CyberKnife collimator of 5mm used in the treatment plan. The mean D(1%), of the brainstem was 10.5± 2.1Gy (P=0.5316) and the mean value of the maximum point dose within the brainstem was 35.6±3.8Gy. This shows the degree of dose fall off within the brainstem. Though the results of the present study are showing superior sparing of brain stem and reasonable of target coverage, it is necessary to execute the treatment plan with greater accuracy in CyberKnife as the immobilization is noninvasive and frameless.

Equivalent normalized total dose estimates in cyberknife radiotherapy dose delivery in prostate cancer hypofractionation regimens.

As the α/ß value of prostate is very small and lower than the surrounding critical organs, hypofractionated radiotherapy became a vital mode of treatment of prostate cancer. Cyberknife (Accuray Inc., Sunnyvale, CA, USA) treatment for localized prostate cancer is performed in hypofractionated dose regimen alone. Effective dose escalation in the hypofractionated regimen can be estimated if the corresponding conventional 2 Gy per fraction equivalent normalized total dose (NTD) distribution is known. The present study aims to analyze the hypofractionated dose distribution of localized prostate cancer in terms of equivalent NTD. Randomly selected 12 localized prostate cases treated in cyberknife with a dose regimen of 36.25 Gy in 5 fractions were considered. The 2 Gy per fraction equivalent NTDs were calculated using the formula derived from the linear quadratic (LQ) model. Dose distributions were analyzed with the corresponding NTDs. The conformity index for the prescribed target dose of 36.25 Gy equivalent to the NTD dose of 90.63 Gy (α/ß = 1.5) or 74.31 Gy (α/ß = 3) was ranging between 1.15 and 1.73 with a mean value of 1.32 ± 0.15. The D5% of the target was 111.41 ± 8.66 Gy for α/ß = 1.5 and 90.15 ± 6.57 Gy for α/ß = 3. Similarly, the D95% was 91.98 ± 3.77 Gy for α/ß = 1.5 and 75.35 ± 2.88 Gy for α/ß = 3. The mean values of bladder and rectal volume receiving the prescribed dose of 36.25 Gy were 0.83 cm3 and 0.086 cm3, respectively. NTD dose analysis shows an escalated dose distribution within the target for low α/ß (1.5 Gy) with reasonable sparing of organs at risk. However, the higher α/ß of prostate (3 Gy) is not encouraging the fact of dose escalation in cyberknife hypofractionated dose regimen of localized prostate cancer.

Dose linearity and monitor unit stability of a G4 type cyberknife robotic stereotactic radiosurgery system.

Dose linearity studies on conventional linear accelerators show a linearity error at low monitor units (MUs). The purpose of this study was to establish the dose linearity and MU stability characteristics of a cyberknife (Accuray Inc., USA) stereotactic radiosurgery system. Measurements were done at a depth of 5 cm in a stereotactic dose verification phantom with a source to surface distance of 75 cm in a Generation 4 (G4) type cyberknife system. All the 12 fixed-type collimators starting from 5 to 60 mm were used for the dose linearity study. The dose linearity was examined in small (1-10), medium (15-100) and large (125-1000) MU ranges. The MU stability test was performed with 60 mm collimator for 10 MU and 20 MU with different combinations. The maximum dose linearity error of -38.8% was observed for 1 MU with 5 mm collimator. Dose linearity error in the small MU range was considerably higher than in the medium and large MU ranges. The maximum error in the medium range was -2.4%. In the large MU range, the linearity error varied between -0.7% and 1.2%. The maximum deviation in the MU stability was -3.03%.

Dosimetric comparison of Linac-based (BrainLAB®) and robotic radiosurgery (CyberKnife ®) stereotactic system plans for acoustic schwannoma.

A dosimetric comparison of linear accelerator (LA)-based (BrainLAB) and robotic radiosurgery (RS) (CyberKnife) systems for acoustic schwannoma (Acoustic neuroma, AN) was carried out. Seven patients with radiologically confirmed unilateral AN were planned with both an LA-based (BrainLAB) and robotic RS (CyberKnife) system using the same computed tomography (CT) dataset and contours. Gross tumour volume (GTV) was contoured on post-contrast magnetic resonance imaging (MRI) scan [planning target volume (PTV) margin 2 mm]. Planning and calculation were done with appropriate calculation algorithms. The prescribed isodose in both systems was considered adequate to cover at least 95% of the contoured target. Plan evaluations were done by examining the target coverage by the prescribed isodose line, and high- and low-dose volumes. Isodose plans and dose volume histograms generated by the two systems were compared. There was no statistically significant difference between the contoured volumes between the systems. Tumour volumes ranged from 380 to 3,100 mm(3). Dose prescription was 13-15 Gy in single fraction (median prescribed isodose 85%). There were no significant differences in conformity index (CI) (0.53 versus 0.58; P = 0.225), maximum brainstem dose (4.9 versus 4.7 Gy; P = 0.935), 2.5-Gy volume (39.9 versus 52.3 cc; P = 0.238) or 5-Gy volume (11.8 versus 16.8 cc; P = 0.129) between BrainLAB and CyberKnife system plans. There were statistically significant differences in organs at risk (OAR) doses, such as mean cochlear dose (6.9 versus 5.4 Gy; P = 0.001), mean mesial temporal dose (2.6 versus 1.7 Gy; P = 0.07) and high-dose (10 Gy) volume (3.2 versus 5.2 cc; P = 0.017). AN patients planned with the CyberKnife system had superior OAR (cochlea and mesial temporal lobe) sparing compared with those planned with the Linac-based system. Further evaluation of these findings in prospective studies with clinical correlation will provide actual clinical benefit from the dosimetric superiority of CyberKnife.